British intelligence agency MI6 is secretly pressuring President Donald Trump to avoid publishing classified information related to the Russia investigation and a surveillance warrant for his former campaign foreign policy adviser Carter Page, according to reports.

On Wednesday, Trump threatened to declassify documents related to the Justice Department’s probe into Russian election interference, claiming that doing so would undermine the efforts of incoming House Democrats to investigate him. He also plans to release information about a Foreign Intelligence Surveillance Act (FISA) warrant that allowed federal investigators to wiretap Page, who has suspected ties to Russia.

The U.K. has warned Trump that declassifying this information would sabotage intelligence collection efforts, according to British newspaper The Daily Telegraph. The revelation raises questions about whether the British were involved in the FBI's investigation into possible collaboration between Trump's campaign and Russia to influence the outcome of the 2016 election.

That investigation began secretly in July 2016, around the same time as the Republican National Convention. It is now being led by special counsel Robert Mueller and his team and has resulted in numerous indictments of both Russians and former Trump associates.

Opponents and allies of Trump alike have asked the president not to release the classified information related to the FISA warrant and the investigation because it could compromise intelligence sources and methods.

The Page warrant issue was first raised in January when the chairman of the House Intelligence Committee, Trump ally Devin Nunes, a Republican from California, began threatening to release a controversial memo alleging that the FBI had not provided adequate information to justify the renewal of the FISA warrant for Page.

The Nunes memo, which was eventually released in early February, claimed that the FBI had failed to disclose that it relied on a controversial dossier compiled by former MI6 intelligence officer Christopher Steele in order to seek the warrant.

Steele had ended his employment with MI6 in 2009 and launched his own private intelligence company, Orbis Business Intelligence. The company was later hired to collect opposition research on the Trump campaign and concluded that Trump had been cultivated as a Russian asset for years. The FBI respected Steele’s reputation enough to brief Trump on the contents of the dossier after he was elected president.

Carter Page, a former aide to President Donald Trump, arrives at the courthouse. Trump plans to release information about a Foreign Intelligence Surveillance Act (FISA) warrant that allowed federal investigators to wiretap Page, who has suspected ties to Russia. Drew Angerer/Getty Images

Trump has repeatedly said that the claims in the dossier are false, and Republicans have argued that the FBI should not have used the Steele dossier, also known as the Russia dossier, to apply for a warrant to surveil a U.S. citizen.

In reality, however, the FBI had reasons to suspect that Page was a Russian agent even before the Steele dossier was drafted. Russian agents struck up a relationship with Page long before he joined the Trump campaign, according to law enforcement. What’s more, the process through which the FBI determines whether a FISA warrant is justified is meant to remain classified. Critics of the president say that it is inappropriate to threaten to release classified information for political purposes.

Trump and the Republicans are bracing for numerous investigations expected be launched after the new Democrat-majority House takes over in January. In August, Axios published a spreadsheet, which the Republicans were circulating on Capitol Hill, listing investigations they believed a Democratic House would launch.

The list included probes into the Trump family's business ties and compliance with the Constitution’s emoluments clause, Trump’s tax returns and the firing of former FBI Director James Comey.

Senator Bob Menendez said he was "disappointed, but not surprised, that the Trump administration has failed once again to prioritize our long-term national security interests or stand up for human rights."